1. Field of the Invention
The present invention relates to a method and a circuit for driving a capacitive load, of the type in a display of a plasma display panel. Each of the cells that constitute the screen of the plasma display panel is a capacitive load for a power source circuit except at the time of gas discharge. A drive circuit that can supply a complicated voltage waveform is desirable, so as to realize a stable display with high quality.
2. Description of the Prior Art
An AC type plasma display panel utilizes a memory function of a dielectric layer that covers the electrodes. Namely, addressing is performed by a line scanning format for controlling charge quantity of the cell in accordance with display data, and then a sustaining voltage Vs having alternate polarity is applied to a pair of the electrodes. The sustaining voltage Vs satisfies the relationship (1) below.
Vfxe2x88x92Vw less than Vs less than Vfxe2x80x83xe2x80x83(1)
Here, Vf is a discharge starting voltage and Vw is a wall voltage between the electrodes.
The application of the sustaining voltage Vs makes a cell voltage Vc (a sum of the applied voltage and the wall voltage, which is also referred to as an effective voltage) exceed the discharge starting voltage Vf only in the cell in which the wall charge exists, and causes the discharge. When the period of application of the sustaining voltage Vs is shortened, an apparently continuous ON state is obtained. Since the cell of the plasma display panel is a binary light emitting element, a gradation tone is reproduced by setting the discharge times of a field for each cell in accordance with the gradation level. A color display is a kind of the gradation display and is obtained by a combination of intensities of three primary colors. In a method of displaying the gradation, a field is made of plural subfields having a weight of the intensity, and the number of total discharge times is set by a combination of on or off for each subfield. In general, an addressing preparation period (that is also referred to as an initialization period) is assigned to each subfield adding to an addressing period and a sustaining period (that is also referred to a display period). At the time of finishing the sustaining period, there are cells with remaining wall charge and cells 10 without the wall charge. Therefore, the charged state of all cells is uniformed in the addressing preparation period so as to improve the reliability of the addressing.
As an addressing preparation process, there is a method of applying a ramp voltage having a small gradient to cells (as being disclosed in U.S. Pat. No. 5,745,086). When the applied voltage increases gradually and reaches the discharge starting voltage, a first discharge occurs. Since the wall voltage is decreased a little by the discharge and the cell voltage drops, the discharge finishes shortly. However, another discharge will occur since the increase of the applied voltage continues. Thus, the discharge repeats in a short period. As the speed of increase of the applied voltage becomes slow, the repeated discharges become a continuous discharge without an interval. The discharge is a weak discharge (a micro discharge) in which the polarity of the wall voltage does not alter, so the light emitting quantity is substantially zero. The preparation process does not affect a contrast. The cell voltage is maintained substantially at the discharge starting voltage Vf by the micro discharge. However, the wall voltage is decreased gradually by every micro discharge. The wall voltage value Vwr at the time when the application of the ramp voltage is finished depends on the discharge starting voltage Vf (Vwr=Vfxe2x88x92Vr). Thus, by generating the micro discharge periodically, the wall charge is adjusted for each cell so as to compensate a dispersion of the discharge starting voltage Vf that can be among cells. Accordingly, the intensity of discharge in the following addressing is uniformed, so that the incidence of address error can be decreased.
A step voltage that increases step by step is more preferable than the ramp voltage that increases continuously for adjusting the charge as the addressing preparation. It is because that the ramp voltage causes the increase of the discharge intensity along with the repeated micro discharge. It is considered that a cause of this phenomenon is a priming effect caused by accumulation of the space charge. Since a variation width of the cell voltage is increased by the increase of the discharge intensity, the wall voltage at the time of finishing the application of voltage can have an error. There is also a problem that an undesired light emission can occur. In contrast to this, the step voltage can make the intensity of the micro discharge uniform by selecting the waveform.
Since the discharge starting voltage Vf of a plasma display panel is approximately 170-200 volts, a step voltage whose maximum voltage is approximately 250-300 volts should be applied. Conventionally, plural bias voltage sources having different output voltages are prepared and one of them is selected to be connected to the electrode by using a switching device, so that the application of a step voltage is performed.
The conventional circuit has a disadvantage in that plural power sources and switching devices are necessary for the number of steps of the step voltage, and so the circuit becomes large when increasing the number of steps. In addition, though the waveform can be changed by setting the control timing of the switching device, the voltage difference between steps and the voltage transition characteristics between steps are fixed.
The object of the present invention is to realize a simple circuit for setting a desired step height in the application of the step voltage. Another object is to set the voltage transition characteristics between steps so as to obtain various waveforms. Still another object is to increase a flexibility of setting a waveform in driving a gas discharge display device.
The present invention utilizes a charge accumulation function of a capacitive load and sets a waveform by controlling on and off of charging current.
According to a first aspect, the present invention provides a method of applying a step voltage to a pair of electrodes for driving a capacitive load. The method includes the steps of providing a current path from a power source to one of the electrodes via a current restricting resistor and a switching path of a semiconductor switching device in order, and transferring charge from the power source to the electrode intermittently by switching control of the semiconductor switching device, so as to increase charge quantity accumulated in a capacitor between the electrodes step by step.
According to a second aspect of the present invention, the voltage between the switching control terminal of the semiconductor switching device and the power source is maintained at a constant value in a period of closing the switching path, so as to make the step to step portion of the waveform of the charge voltage ramp-like shape.
According to a third aspect, the present invention provides a method of adjusting charge by gradually decreasing wall charge of a dielectric that covers the pair of electrodes as a preparation process of addressing to control charge distribution of a screen for driving a gas discharge display device. The method includes the steps of providing a current path from a power source to one of the electrodes via a current restricting resistor and a switching path of a semiconductor switching device in order, and transferring charge from the power source to the electrode intermittently by switching control of the semiconductor switching device, so as to increase charge quantity accumulated in a capacitor between the electrodes step by step.
According to a fourth aspect, the present invention provides a drive circuit for adjusting charge by gradually decreasing wall charge of a dielectric that covers the pair of electrodes as a preparation process of addressing to control charge distribution of a screen in a gas discharge display device. The drive circuit includes a first semiconductor switching device for opening and closing the current path between an output terminal connected to one of the electrodes and a bias potential line, a current restricting resistor inserted between the bias potential line and the semiconductor switching device, a second semiconductor switching device for opening and closing the current path between the output terminal and the ground potential line, and a controller for controlling the first and the second semiconductor switching devices.
According to a fifth aspect, the drive circuit further includes a diode connected to the current restricting resistor in parallel and in the opposite direction, and a capacitor inserted between the terminal of the bias potential line side of the first semiconductor switching device and the ground potential line.
According to a sixth aspect, the drive circuit further includes a variable voltage source for switching the potential of the switching control terminal of the semiconductor switching device.
According to a seventh aspect, the controller includes a memory for memorizing pulse width modulation data used for the switching control.
According to an eighth aspect, the present invention provides a display device includes a drive circuit mentioned above as the fourth aspect and an AC type plasma display panel driven by the drive circuit.